1. Field of the Invention
The present invention relates to disk drives. More particularly, the present invention relates to a disk drive employing momentum based unload during power failure.
2. Description of the Prior Art
FIG. 1A shows a prior art disk drive comprising a head 2 connected to a distal end of an actuator arm 4 which is rotated about a pivot by a voice coil motor (VCM) in order to actuate the head 2 radially over a disk 6. The VCM comprises a voice coil 8 that generates a magnetic flux when energized with current. The magnetic flux generated by the voice coil 8 interacts with magnetic flux generated by permanent magnets (not shown) to generate a torque that rotates the actuator arm about the pivot.
A spindle motor (not shown) rotates the disk 6 while the head 2 accesses a target data track on the disk 6. During a power failure, the head 2 is parked using the back EMF (BEMF) voltage present over the spindle motor windings due to the angular momentum of the spindle and disk assembly. Energy stored in a capacitor may provide additional current to assist the parking operation. In disk drives that employ ramp load/unload, the VCM rotates the actuator arm 4 toward the outer diameter of the disk 6 and onto a ramp 10. Due to torque constraints of the VCM, prior art disk drives typically rely on the momentum of the actuator arm 4 when it reaches the ramp 10 to help carry the actuator arm onto and up the ramp 10. This requires the velocity of the actuator arm 4 to reach a predetermined threshold before it reaches the ramp 10.
FIGS. 1B and 1C illustrate a prior art technique for ensuring the actuator arm 4 reaches the desired velocity before reaching the ramp 10 during the unload operation. As shown in FIG. 1B, when a power failure occurs, the BEMF of the spindle motor is used to move the head 2 to the inner diameter of the disk 6. A constant voltage is applied to the voice coil 8 until a tang 12 coupled to the voice coil 8 contacts an inner arm 14A of a crash stop 16. As shown in FIG. 1C, a constant voltage is then applied to the voice coil 8 to move the head 2 toward the outer diameter of the disk 6 at a constant velocity. This ensures the actuator arm will reach the desired velocity and corresponding momentum to carry the actuator arm 4 onto and up the ramp 10. The outer arm 14A of the crash stop 16 contacts the tang 12 to limit further outward movement once the actuator arm 4 is safely unloaded onto the ramp 10.
Moving the head 2 to the inner diameter of the disk 6 and then to the outer diameter of the disk 6 to ensure the actuator arm 4 reaches the desired velocity results in an inefficient use of power since it is not necessary for the actuator arm 4 to traverse its entire stroke to attain the desired velocity. This increases the overall cost of the disk drive as it requires a more efficient (expensive) VCM to ensure the unload operation finishes before the BEMF voltage generated by the spindle motor dissipates. There is, therefore, a need to improve power efficiency when unloading the actuator arm 4 onto the ramp 10 during power failure.